An electrophotographic printing system typically needs to continually regulate a toning density of an imaging subsystem by periodically reading image density and adjusting various imaging parameters to maintain the desired image density. The imaging subsystem can be adjusted by attempting to print a uniformly toned patch (or target) onto the sheet transport mechanism and then taking one or more process variable measurements of the printed patch.
In order to accurately read the patch, the patch position on the transport mechanism must be accurately tracked from the point of it being printed onto the transport mechanism to the point of it being read by a densitometer sensor. For example, the patch may be printed so that the reflectance (mass deposit) of the developed toner can be measured with a reflectance meter. That information is subsequently used to adjust exposure or toner concentration. In another example, a specific pattern of two or more colors may be printed to evaluate color plane registration for a color printer.
To conduct these process measurements, it is likely that the printing process will be interrupted so that the control images can be printed. For production printers that handle large, high-volume print jobs, interruptions to carry out process control measurements reduces the printer throughput and creates waste.
In some instances cut sheet printers are able to perform these process control steps in an inter-image gap between pages. However, with increasing process speeds the gaps are becoming too small to print the required control images. Moreover, continuous forms printers generally do not have an inter-image gap to use for process control measurements. Printing the control images on the edge of the media or in a designated band may be implemented. Nonetheless, wasted paper and additional steps required to trim off the image that is not part of the print job is a result.